Heretofore, rubber was recovered from guayule plants by the water flotation process which involved parboiling the shrub to help coagulate the latex and minimize loss of latex during wet milling of the shrub in a pebble mill. The parboiling also aided in removal of dirt and leaves. The crude rubber would be recovered via a water flotation process which involved allowing the crude rubber to float on the surface of the water and subsequently collecting the crude rubber by various means.
An improved method involved the fine grinding of a water slurry containing rough ground guayule as ground in a pulping mill to rupture a greater percentage of the cells containing the rubber latex with recovery of crude rubber by the water flotation process. However, the efficiency of the water flotation process does vary with the variety of guayule shrub used as well as the general condition of the shrub. Hence, yield of rubber will vary over a considerable range. In addition, fine grinding in a pulping mill so effectively released the rubber and coagulated the rubber that at times the grinding plates were fouled.
To overcome the variability of the efficiency of the water flotation process, attempts were made to treat the crushed guayule shrub with solvents to effect dissolution of the rubber or resin with subsequent recovery of the rubber or resin from the respective solution. These attempts involved percolation which involved passage of solvent through a bed of guayule shrub to remove the desired rubber by dissolution. Guayule resin can be efficiently recovered by solvent percolation; however, because of the high viscosity of the rubber solution, percolation by gravity flow rapidly compacted the bed of ground shrub resulting in failure of the gravity percolation process.
Still another method of recovering the rubber or resin involved subjecting the plant materials to a simultaneous action of compressive and shear forces under nonaqueous conditions, wherein said forces are sufficient to reduce said fiberous plant material to comminuted fiberous matter and, thereby, release polymeric hydrocarbon substances. The comminuted fiberous matter and polymeric hydrocarbon substances would cohere into a plastic mass. The plastic mass was then shaped into particles, and polymeric hydrocarbon substances extracted therefrom with a solvent. However, this method, as set forth in U.S. Pat. No. 4,136,131 related to open milling of the guayule shrub as well as size reduction in an extruder and thus would expose the ground shrub to the deleterious effects of air contact unless the operations are conducted in an inert atmosphere.
Moreover, this procedure to form preshaped particles of comminuted guayule shrub involved an additional step since the preshaped particles are then extracted via a percolation process.
Many variations of the percolation and immersion processes are possible but no known previous procedure reports the simultaneous recovery of rubber and resin via dissolution in an appropriate solvent.
Another method of recovering rubber or resin was to immerse the crushed guayule shrub in a solvent to effect dissolution and subsequently recovering the solution of rubber or solution of resin. This procedure would avoid the compacting of the shrub bed experienced with solvent percolation to recover rubber. This procedure is commonly called an immersion process.
U.S. Pat. No. 2,390,860 relates to using for extraction a resin solvent as a resin solvent with a small amount of a rubber solvent such that the mixture is not a rubber solvent. This patent is not pertinent in that it relates to using a material obtained via a water floatation process. U.S. Pat. No. 2,572,046 relates to an extraction method having a separate water-miscible, organic solvent phase, and a separate hydrocarbon solvent phase. Hence, in relating to two different phases, it is not pertinent. None of these patents suggests the use of a monophase mixture of solvents, or to the formation of a slurry therewith and the fine grinding thereof.